A buoy is a floating or marker that is placed in the sea. It can have many different purposes, which determine whether the buoy is anchored (stationary) or allowed to drift. The word is most commonly said ([bɔɪ]) (as in buoyant), but in American English it is often pronounced [ˈbu.i].
One of the many purposes are keeping the people safe from strong currents. Buoys mark the end of the safety zone, if people swim past the buoys they will be likely to be dragged out to sea.